Ski, the 563 is just as good as its specs. Power output advertising is governed by FTC(Federal Trade Commission)regulations and this is about the only area in audio equipment where you can be confident that what you see is what you get. The 563 should have plenty of power capacity for the speakers that you have. It doesn't have some of the most up-to-date features, but since you're working with a limited budget for audio equipment, there's certainly no urgency in making a change.
The speakers that you mention, the 7800 and 7900, are certainly close enough that either can be used together as surrounds. If you have at least 4' behind your listening position where you could put the 7900s, you'll get some benefit from the back surround sound. Of course with those small woofers there's practically no bass output below about 100Hz and you're missing out on a big chunk of sound. When your budget permits you should get better speakers, especially for the front, and a sub to fill in that missing bass.
